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3.2.4Additional lessons

IUPAC Nomenclature — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 61 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5Building a Name

3.2.4 IUPAC Nomenclature
Stem
  • The longest carbon chain sets the stem: meth (1), eth (2), prop (3), but (4), pent (5), hex (6).
Suffix
  • The principal functional group gives the ending, such as -ane, -ene, -ol, -al, -one or -oic acid.
Prefix
  • Substituents are named as prefixes, such as methyl, ethyl, chloro or bromo, and listed alphabetically.
Locants
  • Numbers show the position of branches and groups, and the chain is numbered to give the lowest number to the principal group.
Multipliers
  • Di-, tri- and tetra- show how many identical groups are present, but they are ignored when alphabetising.

Part 2 of 5Naming Step by Step

1
Identify the principal functional group, which decides the suffix and where numbering starts.
2
Find the longest chain containing that group and use the matching stem.
3
Number the chain from the end that gives the lowest locant to the principal group, then to other substituents.
4
Name each branch or halogen as a prefix with its locant, listing prefixes in alphabetical order.
5
Assemble the name using hyphens between numbers and letters and commas between numbers, for example 2-bromo-3-methylbutane.
StructureName
CH₃CH₂CH₂OHPropan-1-ol
CH₃COCH₃Propanone
CH₃CH₂COOHPropanoic acid
CH₃CH(Cl)CH₃2-chloropropane

Part 3 of 5Special Cases

Special Cases
Alkenes
  • The double bond is numbered with the lower locant, giving names such as but-2-ene.
Benzene rings
  • A benzene ring with a side chain is named as phenyl- or as a substituted benzene such as methylbenzene.
Esters
  • The alcohol part is named first as an alkyl group, then the acid as -oate, so CH₃COOCH₂CH₃ is ethyl ethanoate.
Amines
  • Primary amines are named as -amine, so CH₃CH₂NH₂ is ethanamine.

Part 4 of 5Worked Example

Question Name CH₃CH(CH₃)CH₂CH(OH)CH₃.

  1. The principal group is –OH, so the name ends in -ol.
  2. The longest chain containing the carbon with –OH has five carbons: pentan-ol.
  3. Number from the end nearest to –OH: the –OH is on C2 and the methyl branch is on C4.
  4. Assemble the prefix and locants.

Answer 4-methylpentan-2-ol

Part 5 of 5Exam Checklist & Tips

3.2.4 – Key Points for the Exam

  • IUPAC The international union that sets agreed rules for systematic names.
  • Locant A number showing the position of a substituent or functional group.
  • Substituent An atom or group that replaces hydrogen on the main chain.
  • Principal functional group The group that determines the suffix and the numbering.

Common Mistakes

  • Choosing a chain that is not the longest one.
  • Numbering from the wrong end so that the principal group does not get the lowest locant.
  • Using a comma between letters and numbers instead of hyphens.
  • Writing ethyl before methyl, forgetting alphabetical order.

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